Previous posts have dealt with the storage levels of hydro reservoirs in Tasmania, a subject of great interest due to them being at very low levels at the same time as the non-availability of the Basslink interconnector. This post attempts to shed some light on the question of why the reservoir levels are so low, by looking in detail at what happened during the 2015 “rainy” season, May to October.
In the autumn/winter rainy season the large reservoirs are normally allowed to be replenished, with most electricity coming from the small reservoir and run-of-river hydro generators. The replenishment allows the large reservoirs to take over the generation of most electricity during the “dry” season, spring/summer, in particular during periods of drought.
The water inflow to the reservoirs in 2015 can be seen from the following figure, produced by Hydro Tasmania:
Water inflow was abnormally low in springtime, especially from October to December 2015, but was mostly higher than average earlier in the year, especially in May. The healthy water inflow to August 2015 should have allowed substantial replenishment of the large hydro reservoirs, but the data on water storage and electricity generation tell a different story.
The significance of the reservoir storage data has been discussed in previous posts and is shown again in the following figure:
The following set of figures shows the daily average number of MW generated by each hydro basin, and by wind and gas generators, for each day from 2nd May to 31st October 2015, with the total output over all generators shown in the last one:
Gordon Basin/Power Station
This large reservoir generator averaged around 200 MW output during most of the 6 month period, and produced a total of 880 GWh of energy, whilst its reservoir level remained almost constant. The figure of 880 GWh is important as it indicates roughly how much storage the reservoir might regain in 2016, if the power station produces no electricity during the 2016 “wet” season. The Gordon reservoir could have been replenished by 880 GWh in 2015 if it had been allowed to, there is plenty of reserve generating capacity on the mainland between May and September (a low demand period), with Basslink available at that time to export it to Tasmania.
Esk Basin (Poatina Power Station)
This basin also produced an average of around 200 MW, and a total of 800 GWh of energy, with a small rise in reservoir storage. The Esk reservoir could have been further replenished by 800 GWh in 2015 if it had been allowed to.
King Basin (John Butters Power Station, 143 MW nameplate)
This small reservoir system produced mostly a steady average output of around 100 MW, before falling away in the dry months of September and October.
This small reservoir system has 484 MW of nameplate capacity hydro generators, which produced at almost full capacity until the dry period at the end.
This basin has intermediate sized storage and managed to produce around 200 MW until the dry period at the end, whilst at the same time increasing its water storage significantly.
This basin has mostly run-of-river generators, which produced around 150 MW on average until the dry period at the end.
The nameplate capacity of the two wind farms in Tasmania is 308 MW, which came close to being achieved on several days, but the “roaring forties” were often the “whispering forties”, with very low wind output on many days.
The gas generators (Bell Bay and Tamar Valley) were resting during this period, before their starring role after the loss of Basslink in December 2015.
The plot of total generation reveals the mistake made by Hydro Tasmania: the large reservoir (Gordon and Esk) systems produced more electricity than was needed by Tasmania between May and August, so in effect a prudent replenishment of their reservoirs when it was raining/snowing was traded for income from the export of electricity to the mainland. The Basslink capacity was there at the time to allow zero electricity output from Gordon and Esk. The water storage history suggests that the same mistake was made at Lake Gordon in the 2014 “wet” season.
At the (dry) end of the period there was a substantial shortfall in local supply, with the balance coming from Basslink imports, until its loss in December, which forced further depletion of reservoirs.
The 2015 “wet” season figures of 880 and 800 GWh of energy production for Gordon and Esk provide estimates of how much gain in water storage is achievable during the 2016 “wet” season. The actual gains in water storage in 2016 will set the figures for the maximum amount of electricity that can be generated from those reservoirs in the subsequent “dry” season, on the assumption that a decision is made to stop further depletion of the reservoirs (see the previous post).
Future posts here will report on what is happening in Tasmania in 2016.